Routers and switches (network devices) forward packets from an ingress interface to an egress interface. Inside the network device a packet passes through several packet forwarding hardware blocks such as, for example, a packet processing engine, a forwarding engine etc. where each block has input/output interfaces and the blocks are connected via their interfaces. Generally different network devices include different sets packet processing hardware blocks. The operating system of a particular network device includes tools accessible by the command line interface (CLI) to access counters that keep track of the number of packets received and transmitted at each interface of the packet processing hardware blocks.
If it is discovered that the some ingress packets does not egress at the expected interface, then it may be necessary to perform a debugging procedure on the network device. The result of the debugging procedure typically is to identify the exact point or exact block, in as much detail as possible, where the packets might be getting dropped.
A technician having detailed knowledge of the internal architecture and operation of the network device, the internal hardware logic blocks, the connections among internal interfaces, and path expected to be taken by a given packet may use one or more of the hidden CLI to investigate where the packets might be getting dropped. However, in general, users do not have such detailed familiarity and accessibility of the devices being used at their facility, and such debugging based on intimate knowledge of the device inner working is not possible.